Why are some received calls showing as UNKNOWN - does not show the number from the caller?

[AC Question]

Some received calls are showing "UNKNOWN" - does not show the number from the caller

I Have a Samsung Galaxy S5 with Android 5

Some Received calls are showing "UNKNOWN"


I have a Samsung Galaxy S5 with Android 5.0

Some Incoming Calls are showing "UNKNOWN"

Does not display the caller's phone number.

This happens with a few phone calls.

Some calls I receive show the caller number, others show the name of the caller, and others show UNKNOWN

These people that I receive calls as "UNKNOWN" do not activated the feature to block the sending of your number, so it should not show on my phone as Unknown.

It is a big problem. How to solve ?